1. Field of the Invention
This invention relates generally to an apparatus and method utilizing microstrip branch-lines for generating output signals of different phases. More particularly, this invention relates apparatus and method utilizing decoupled microstrip branch-lines for generating output signals of different phases wherein the prediction of the output signal characteristics is simplified with improved accuracy.
2. Description of the Prior Art
As more portable personal communication systems are made available as a result of recent progress in the semiconductor technology and packaging engineering, one difficulty often encountered in the efforts in device miniaturization is the interference of the electromagnetic fields between various circuits. This concern for interference becomes more critical when the dimensions of these circuits become smaller and the transmission lines are being formed with shorter distances from each other. On the one hand, these interference often impose limitations on the performance level, such as the bandwidth, of a device. On the other hand, since the interference is a complicated physical phenomena, the effects of caused by the interference are difficult to predict thus causing a great deal of design uncertainties and inaccuracies.
One specific example of such device is a hybrid coupler comprises microstrip branch-lines of different lengths to generate output signals of different phases. Theses couplers or phase-shifting circuits are often being employed in the microwave circuits. For the purpose of shifting the phase by a 90.degree. or 180.degree., the conventional methods utilize a quarter wave length or half wave length transmission lines. However, these transmission lines become too long for implementation in the integrated circuits (ICs). In order for these coupler to be implemented in a monolithic microwave integrated circuit (MMIC), various techniques are used to reduce the dimensions of the circuit elements and the length of the transmission lines. A lumped-element approach is disclosed by Caulton et al. in `Status of Lumped Elements in Microwave Integrated Circuits--Present and Future` (IEEE Transaction, Microwave Theory and Technology, Volume MTT-19, pp. 588-99, July 1971), which uses spiral inductors and lumped capacitors. This approach must use an empirical design method with precise inductor model derived from careful measurements of test elements. The empirical method becomes very complex at higher frequencies and thus is not practically useful for most of the modern communication applications. Gupta et al. disclose a quasi-lumped element branch-line coupler in `Quasi-lumped Element 3 and 4-ports Networks for MIC and MMIC Applications` (`IEEE MTT-S International Microwave Symposium Digest`, 1984, pp.409-411) which uses lumped capacitors and short-circuited transmission lines as inductor elements. The coupler is free from the uncertainties caused by the lumped inductors but the layout of these circuits is inconvenient for tight integration.
Hirota et al. disclose a size reduction circuit technique for constructing a hybrid coupler in `Reduced-size Branch-line and Rat-Race Hybrids for Uniplanar MMIC's` (IEEE Transaction on Microwave Theory and Techniques, Volume 28, number 3, March 1990) which utilizes combinations of short high impedance transmission lines and shunt lumped capacitors. The size of the coupler is reduced with this technique that a 3 dB branch line coupler can be developed using transmission lines of one-eighth or one-twelfth of a wave length. The length reduction is accomplished by compensating the loss of the inductance and capacitance due to the shortening of the transmission line by increasing the `characteristic impedance` to offset the inductance loss and by adding lumped capacitors to offset the capacitance loss.
FIGS. 1A and 1B show the circuit diagrams of a branch-line hybrid coupler wherein FIG. 1A is a traditional coupler 10 and FIG. 1B shows a coupler 20 of reduced size by the use of the technique as disclosed by Hirota et al. Referring to FIG. 1A, the length of the branch lines, i.e., 12-1, 12-2, 12-3, and 12-4 on each side of the traditional coupler 10 is quarter wave length (.lambda.g/4) and the characteristic impedance of these branch lines are Z.sub.0 =50 .OMEGA. for branch lines 12-1 and 12-3, and Z.sub.0 /.sqroot.2=35 .OMEGA. for branch lines 12-2 and 12-4. The size of the coupler disclosed by Hirota et al., shown in FIG. 1B, is reduced by the use of branch lines with higher characteristic impedance Z.sub.0. By increasing the characteristic impedance to 70.7 ohms, the length of the branch lines of 22-1 and 22-3 is reduced to one-eighth of a wavelength (.lambda.g/8) and the length of the branch lines 22-2 and 22-4 is reduce to one-twelfth of the wavelength (.lambda.g/12).
As the dimension of the hybrid coupler 10 is reduced, the distances between the branch lines are also reduced. A major difficulty in design of the coupler to be operated over a variety of frequency ranges and electromagnetic field variations, such as an MMIC circuit, is to accurately predict the circuit responses under these conditions. The difficulty of accurate prediction is further compounded by the interference between the branch lines. Particularly, when the distances between these lines become smaller, the effects of the interference become more significant and may even dominate the operation characteristics of a coupler. Due to the fact that the interference of the electromagnetic field among branch lines is a complicated phenomenon and a close form analytical solution by solving a set of equations governing the dynamics of the electromagnetic fields is often not feasible. A designer of the circuits often needs to apply numerical analyses by `running` computer programs in order to determine circuit parameters to satisfy performance specifications. Frequently, the output of the computer analyses may not be sufficiently accurate and fine tuning of the circuit may be required by adjusting the circuit configuration, e.g., length of different branch lines, or circuit elements, e.g., resistors, capacitors, etc., before the design of a circuit system, e.g., a coupler, can be finalized.
Even with the more elaborate design processes, which often increases the time and cost of the circuit design cycle, the operation of such a circuit system may still be limited by the interference. The response of a circuit system may be unstable or unpredictable in certain operation range, e.g., a certain frequency range, due to the interference. Thus the electromagnetic field interference among the circuit elements not only causes the design processes to be more complicated and costly, it may also impose undesirable operational limitations on the circuit system thus hindering its capacity to achieve higher system performance.
Therefore, there is still a demand in the art of circuit design and manufacture of MMIC and other IC devices, particularly for portable devices for application to higher bandwidth ranges, an improved circuit configuration and methodology to overcome this difficulty imposed by the electromagnetic interference among circuit elements.